Influenza (the 'flu')

Influenza is a disease caused by the influenza virus. Symptoms of influenza include sudden onset of headache,chills and cough followed by a fever, appetite loss, muscle aches and tiredness. Influenza is a cause of outbreaks in long-term care homes. For more information, see the health unit's fact sheet on "Cold vs. Flu".

The following graph shows the number of influenza cases in Simcoe Muskoka between 1999-00 and 2016-17. There have been between 28 and 691 influenza cases every season since 1999-00. In the 2016-17 flu season, 691 cases of flu were reported in Simcoe Muskoka. Increased testing and more sensitive tests may partially explain the increased incidence in recent years. In 2014-15, the strain in the vaccine for influenza A(H3N2) was not an exact match to the circulating strain, which is likely a contributing factor to the increase in that season.

While cases of influenza A and influenza B are generally reported every year, influenza A is usually the main circulating subtype. Influenza B made up a larger percentage of cases in the 2011-12 flu season and the 2013-14 season. In the 2016-17 flu season, 88% (609) of influenza cases in Simcoe Muskoka were influenza A, while 12% (82 cases) of influenza cases were influenza B. No cases were positive for both flu A and flu B.

The following graph shows the incidence rate of influenza in Simcoe Muskoka and Ontario between 1999-00 and 2016-17. The Simcoe Muskoka rate is comparable to the Ontario rate. However, the three most recent seasons have had higher rates locally than at the provincial level. There is some variability in both rates, which means that there are many peaks and valleys and it is difficult to determine whether an increase is significant or whether it is within the expected range for this disease. This is also because there are so many factors that affect the number of reported influenza cases. For the 2016-17 flu season, the Simcoe Muskoka rate was 124.1 cases per 100,000 population and the Ontario rate was 89.7 cases per 100,000 population.

By Age Group

The following graph shows the age-specific incidence rate of influenza in Simcoe Muskoka between 2006-07 and 2016-17. The age-specific rates are influenced by which subtype and strains of influenza are circulating in each season. For example, the dominant strain in the 2014-15 and 2016-17 seasons was influenza A(H3N2) which generally infects and affects people who are 75 years and older more severely.

By Sex

The following graph shows the sex-specific incidence rate of influenza in Simcoe Muskoka and Ontario between 2005-06 and 2016-17. The incidence rate of influenza is similar for males and females. The increased incidence in females in 2014-15 and 2016-17 is likely because females make up a larger proportion of older adults. The dominant strain in the 2014-15 and 2016-17 seasons was influenza A(H3N2) which generally infects and affects people who are 75 years and older more severely.

Technical Notes

There are many factors that influence how many cases are reported to the health unit, as explained on the Infectious Diseases page.